A knock that appears exactly at the moment of a gear change is misleading: your hand is on the lever, so the gearbox must be to blame. In practice the gearbox rarely knocks — far more often the sound is born where the drive changes: in the engine mounts, in the driveshaft joints, in the fastenings. The gear change is only the trigger, not the cause. Here is what actually knocks, how to narrow it down in one drive, and how the repair is decided.
What happens during a shift
When you press the clutch and select another gear, the torque falls to zero for a fraction of a second and then returns. The engine on its flexible mounts has time to rock one way and come back, the driveshafts unload and load again, and clearances in the joints and fastenings are taken up. While the mount rubber is springy and the play is within normal, all of that happens silently. As soon as one element wears, the same jolt becomes a distinct impact.
Hence a simple search logic: look not in the gearbox but among the parts that hold the engine and carry drive to the wheels. The shift mechanism — rods, linkage, cables — can rattle too, but its sound is different: it comes straight from under the lever, is felt through your hand, and does not depend on whether the car is under load or coasting.
The main sources
| Source | Character of the sound | When it shows |
|---|---|---|
| Engine or gearbox mount | Dull thump into the floor, felt in the body | First, reverse, sharp lift-off |
| Inner CV joint, driveshaft | Metallic knock, sometimes with a lurch | Pulling away, launching under load |
| Shift linkage and cables | Rattle and clicks under your hand | As the lever moves |
| Suspension play, anti-roll bar links | The knock matches the body’s rocking | Over bumps and on a throttle blip |
| Exhaust system | A ringing clank from below | Any change of drive |
Most often the engine mount wins: the rubber-to-metal block delaminates or tears over time and the engine starts moving several millimetres more than intended. That is why the knock is felt through the whole body rather than the lever — signs of worn engine mounts. Similar sounds from the clutch are covered in signs of a worn clutch.
What to check yourself
- The neutral test. Start the car, release the clutch in neutral and listen. An even whine with no knock is a separate story, covered in gearbox whine in neutral.
- A throttle blip while parked. In neutral, add throttle sharply and release. The engine lurches; if there is a dull thump, it is almost certainly a mount. The gearbox takes no part in that test at all.
- Pulling away without throttle. On a level area, release the clutch at idle. A distinct metallic click on one side of the car hints at a driveshaft or its joint — signs of a worn outer CV joint.
- A helper outside. Have someone stand to the side while you repeat the tests: the difference between a sound from under the bonnet and one from under the floor is far clearer outside than in the cabin. The general catalogue of such sounds is on the symptom page knocking when accelerating.
How the repair is decided
The spread is wide because access to the mounts depends heavily on the model: on one car a mount is changed in half an hour, on another the engine has to be supported and half the ancillaries removed.
Two things worth asking. Which mount was found to be moving and how — a mount is judged under load, not by eye. And whether the mount bolts are single-use stretch bolts, which many are: reusing them is a known reason a fresh mount tears again within months.
If the knock is still occasional and you are unsure whether it comes from under the bonnet or under the floor, record it in the Pro-Stuk app during a throttle blip and while pulling away. The app matches the recording with your answers and shows the likely causes with percentages — which makes the conversation at a workshop noticeably shorter.
